In recent years, from the viewpoint of complying with greenhouse gas emissions regulations in association with measures to tackle global warming, reductions in the weight of automobile bodies are being sought with the aim of improving the fuel consumption of automobiles. In addition, because it is necessary to ensure collision safety, the application of high-strength steel sheets is increasing. In particular, recently there is an increasing need for ultra-high strength steel sheets having a tensile strength of 980 MPa or more, preferably 1180 MPa or more. Further, there is a demand for an ultra-high strength hot-dip galvanized steel sheet whose surface has undergone hot-dip galvanization for use at regions in an automobile body that require rust preventing properties.
One of methods for forming vehicles or members of automobiles using such high-strength steel sheets is a bending method, such as press forming. In general, the bendability tends to deteriorate as the strength of a steel sheet is increased. Therefore, there has been a problem in that when a high-strength steel sheet is subjected to bending, fissures (cracks) occur within the steel sheet at a deformed part.
It is known that (a) the degree of difficulty for necking to occur, and (b) the degree of difficulty for cracking (voids) to occur inside the steel sheet are important as factors that govern the bendability of a high-strength steel sheet. It is considered that this is because, in a steel sheet having a low degree of elongation, necking occurs during bending and deformations are localized, and consequently the bendability deteriorates.
During bending of a steel sheet, a large tensile strain in the circumferential direction arises at a surface layer portion of a bent outer peripheral surface, and a large compressive strain arises at a surface layer portion of a bent inner peripheral surface. Therefore, the bendability of an ultra-high strength steel sheet is significantly affected not only by the steel micro-structure inside the steel sheet, but also by the steel micro-structure of the surface layer portion of the steel sheet. Specifically, it is known that by making the surface layer portion of the steel sheet a soft layer, localization of deformations that arise in the steel sheet surface during bending is lessened and bendability is improved. Patent Documents 1 to 4, that are described hereunder, disclose inventions in which the aforementioned technique is applied to improve the bendability of an ultra-high strength steel sheet.
Patent Document 1 discloses technology that relates to a hot-dip galvanized steel sheet or a galvannealed steel sheet having a tensile strength of 1180 MPa or more in which the bendability is improved by dissolving Zn in a surface layer portion of the steel sheet and softening the surface layer portion of the steel sheet, and furthermore, making the metal micro-structure constituting the steel sheet a micro-structure that mainly consists of martensite and bainite.
Patent Documents 2 and 3 disclose technology relating to an ultra-high strength cold-rolled steel sheet that, by controlling the atmosphere during continuous annealing to an oxidizing atmosphere to cause a decarburized layer to form on an outer layer of a steel sheet, improves the bendability by separately forming a soft layer that mainly consists of ferrite as the outer layer and a hard layer that mainly consists of martensite and bainite as an inner layer.
Patent Document 4 discloses technology relating to a high-strength cold-rolled steel sheet that, after heating a steel sheet, sprays water onto the surface to cool a surface layer portion and thereafter uniformly cools from the outer layer of the steel sheet to the interior to thereby vary the cooling patterns for the surface layer portion and the interior of the steel sheet and separately form a soft layer that mainly consists of ferrite in the outer layer and a hard layer that mainly consists of a low-temperature transformation phase in an inner layer to thus improve the bendability.